Gas separation is important in various industries and can typically be accomplished by flowing a mixture of gases over an adsorbent that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. One of the more important gas separation techniques is pressure swing adsorption (PSA). PSA processes rely on the fact that under pressure gases tend to be adsorbed within the pore structure of the microporous adsorbent materials or within the free volume of a polymeric material. The higher the pressure, the more gas is adsorbed. When the pressure is reduced, the gas is released, or desorbed. PSA processes can be used to separate gases in a mixture because different gases tend to fill the micropore or free volume of the adsorbent to different extents. If a gas mixture, such as natural gas, for example, is passed under pressure through a vessel containing polymeric or microporous adsorbent that fills with more nitrogen than it does methane, part or all of the nitrogen will stay in the sorbent bed, and the gas coming out of the vessel will be enriched in methane. When the bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed nitrogen. It is then ready for another cycle.
Another important gas separation technique is temperature swing adsorption (TSA). TSA processes also rely on the fact that under pressure gases tend to be adsorbed within the pore structure of the microporous adsorbent materials or within the free volume of a polymeric material. When the temperature of the adsorbent is increased, the gas is released, or desorbed. By cyclically swinging the temperature of adsorbent beds, TSA processes can be used to separate gases in a mixture when used with an adsorbent that selectively picks up one or more of the components in the gas mixture.
Adsorbents for PSA systems are usually very porous materials chosen because of their large surface area. Typical adsorbents are activated carbons, silica gels, aluminas and zeolites. In some cases a polymeric material can be used as the adsorbent material. Though the gas adsorbed on the interior surfaces of microporous materials may consist of a layer only one, or at most a few molecules thick, surface areas of several hundred square meters per gram enable the adsorption of a significant portion of the adsorbent's weight in gas.
Different molecules can have different affinities for adsorption into the pore structure or open volume of the adsorbent. This provides one mechanism for the adsorbent to discriminate between different gasses. In addition to their affinity for different gases, zeolites and some types of activated carbons, called carbon molecular sieves, may utilize their molecular sieve characteristics to exclude or slow the diffusion of some gas molecules into their structure. This provides a mechanism for selective adsorption based on the size of the molecules and usually restricts the ability of the larger molecules to be adsorbed. Either of these mechanisms can be employed to selectively fill the micropore structure of an adsorbent with one or more species from a multi-component gas mixture. The molecular species that selectively fill the micropores or open volume of the adsorbent are usually referred to as the “heavy” components and the molecular species that do not selectively fill the micropores or open volume of the adsorbent are usually referred to as the “light” components.
An early teaching of a PSA process having a multi-bed system is found in U.S. Pat. No. 3,430,418 wherein a system having at least four beds is described. This '418 patent describes a cyclic PSA processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) co-current depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower pressure; (4) purge; and (5) repressurization. The void space gas released during the co-current depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure. Another conventional PSA processes using three sorbent beds is disclosed in U.S. Pat. No. 3,738,087. Conventional PSA processes are typically able to recover only one of the key components (i.e., light or heavy) at high purity and are unable to make a complete separation and separate both components with a high recovery. The light component usually has a low recovery factor. Recovery of the light component usually drops even lower when the feed gas is introduced at higher pressures (i.e., pressures above 500 psig).
For the recovery of a purified strongly adsorbed “heavy” component, an additional step is usually necessary, namely, rinsing of the bed with a heavy component to displace the light component from the bed prior to depressurization. The rinsing step is well known in the art. The problems associated with these processes are the following: (a) if the rinsing is complete and the light component is completely displaced from the bed, substantially pure heavy component can be obtained, but the adsorption front of the heavy component breaks through to the light component and the latter cannot be recovered at high purity; (b) if the displacement of the light component is incomplete, the typical concentration profile of the heavy component in the bed is not optimum and as such the bed is depressurized countercurrently to recover the heavy key component at the feed end, the light component still present in the bed reaches the feed end very rapidly and the purity of the heavy component drops. Therefore it is not practical in the prior art to obtain both key components at high purity in a single PSA unit.
The faster the beds perform steps to complete a cycle, the smaller the beds can be when used to process a given hourly feed gas flow. Several other approaches to reducing cycle time in PSA processes have emerged which use rotary valve technologies as disclosed in U.S. Pat. Nos. 4,801,308; 4,816,121; 4,968,329; 5,082,473; 5,256,172; 6,051,050; 6,056,80; 6,063,161; 6,406,523; 6,629,525; 6,651,658 and 6,691,702. A parallel channel (or parallel passage) contactor with a structured adsorbent is used to allow for efficient mass transfer in these rapid cycle pressure swing adsorption processes. Approaches to constructing parallel passage contactors with structured adsorbents have been disclosed in US20060169142 A1, US20060048648 A1, WO2006074343 A2, WO2006017940 A1, WO2005070518 A1, and WO2005032694 A1.
In a parallel channel contactor, the adsorbent lines the wall of the flow channel which can be formed from the space between parallel plates or the open path through a duct or tube. When parallel plates are used to form the parallel channel, a spacer may be present in the space of the parallel channel. An example of a spacer-less parallel passage contactor as provided in US20040197596 A1 and an example of a parallel passage contactor with a high density adsorbent structure is given in US20050129952A1. In all cases, the adsorbent used to line the parallel channel contains both mesopores and macropores.
Mesopores and macropores are known in the art to improve the mass transfer characteristics of adsorbents used in either a parallel channel contactor or conventional packed bed contactors. Improvements in mass transfer characteristics from the presence of mesopores and macropores in conventional packed bed contactors have been widely discussed. See for example U.S. Pat. Nos. 6,436,171 and 6,284,021. Improvements in mass transfer characteristics from the presence of mesopores and macropores in parallel channel contactors are discussed in EP1413348 A1. As such, the prior art teaches that a large number of mesopores and macropores are needed in an adsorbent particle or layer of adsorbent in order to have mass transfer characteristics good enough to operate a pressure swing adsorption cycle. The inventors hereof have unexpected found that adequate mass transfer characteristics can be attained without a significant amount of mesopores and/or macropores providing easy access to the micropore structure in the adsorbent where selective separation occurs.
While there are various teachings in the art with respect to new adsorbent materials, new and improved parallel channel contactors, and improved rapid cycle PSA equipment, none of these to date present a viable solution to the problem of producing good recovery of the light component and purity when the feed gas is at very high-pressure. This is a critical issue since natural gas is often produced at high pressures (500-7000 psi) and methane acts as a light component in the adsorption process. Many gas fields also contain significant levels of H2O, H2S, CO2, N2, mercaptans and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases H2S and CO2 be removed from natural gas as possible. In all natural gas separations, methane is a valuable component and acts as a light component in swing adsorption processes. Small increases in recovery of this light component can result in significant improvements in process economics and also serve to prevent unwanted resource loss. It is desirable to recover more than 80 vol. %, preferably more than 90 vol. % of the methane when detrimental impurities are removed. While various processes exist for removing CO2, H2S, and N2 from natural gas there remains a need for processes and materials that will perform this recovery more efficiently, at lower costs, and at higher hydrocarbon yields, particularly at higher methane yields.
Similarly, for other gaseous feed streams, the prior art describes several ways to recover high amounts of the heavy components in a heavy component rich “reject” stream, but cannot achieve as high a recovery of the light components in the light component rich product stream. This difference in recoveries becomes greater as the feed pressure increases.